Not Applicable
Not Applicable
1. Field of the Invention
This invention pertains generally to the fabrication of multi-layered printed circuit boards, and more particularly to method for attaching copper and steel sheets for use in the manufacture of printed circuit boards, as well as to laminated copper/steel sheets fabricated according to the attachment method.
2. Description of the Background Art
Printed circuit board (PCB) lay-up panels are widely used in the electronics industry, and a number of approaches have been taken to laminate PCB lay-up panels. In early stages of development, PCB lay-up panels were laminated using presses similar to those used in the wood industry for laminating, for example, sheets of plywood. During manufacture, steam or electric power was used to heat hydraulic-driven presses to temperatures exceeding 350xc2x0 F. The panel components in the presses were submitted to pressures between 300 psi and 500 psi at 350xc2x0 F. for approximately one hour to achieve proper lamination. With the early methods, highly polished and precision ground stainless steel plates approximately 0.062 inches thick were used to separate each panel within a press opening. Typically, a T-304 full hard alloy or equivalent material was used for these 0.062 stainless steel separator plates. A regular problem, however, was that these stainless steel 0.062 separator plates required cleaning or scrubbing to remove debris after every use and periodically needed to be resurfaced to remove dents and scratches due to handling and use. Eventually, the plates had to be replaced.
During the late 1980""s, the introduction of vacuum assisted presses permitted the use of lower pressures during the lamination cycle. The pressures used in vacuum assisted presses typically ranged from approximately 150 psi to 250 psi, as opposed to the 300 psi to 500 psi range used in the hydraulic steam driven or electric presses. With vacuum assisted presses, aluminum separator sheets ranging in thickness from 0.007 to 0.015 inches were tested and then used extensively. Test results published during that time indicated that thin aluminum separator plates far exceeded the performance of steel plates for laminating PCB panels. These thin aluminum separator sheets were discarded after the lamination process, thus eliminating the need for expensive steel plate cleaning and handling operations and the frequent and large capital investment needed to replace the steel plates.
The alloy used for aluminum separator plates is typically 3000 series (e.g. 3003, 3004, 3105 or equivalent) with a H19 hardness designation, which is identical to the alloy used to make aluminum beverage cans. The process using thin aluminum separator sheets along with low pressure from vacuum assisted presses has worked well for typical 4 layer to 6 layer PCB""s with circuit lines of approximately 0.008 inches in width and approximately 0.008 inches apart. A typical configuration in a press opening would be a steel plate on top and bottom of the stack with thin aluminum sheets separating each PCB panel. The rate of production in these vacuum assisted presses increased to about 10-14 PCB panels per typical 1xc2xd inch press opening from the 6 to 8 PCB panels achieved using 0.062 stainless steel sheets.
Technological advancements, however, have driven a need for PCBs to have denser circuitry. This means that circuits must have finer lines (less than 0.006 inches wide) and closer spacing between circuit lines (less than 0.006 inches). Denser surfaces on a PCB permit a higher quantity of electrical components to be mounted thereon, thus enabling faster information processing and greater miniaturization of electronic hardware. These greater technological demands have made the surface quality of the laminated circuit board panels even more critical. Problems such as surface roughness and image transfer that also previously existed, have now become critical issues that require resolution, since any minute bump on the surface of the aluminum sheet will be transferred to the top surface of the board necessitating scraping the board and reworking the PCB fabrication process.
To prevent and minimize scrap and rework due to image transfer and surface quality problems, almost every press configuration used today employs 0.062 stainless steel plates (usually T-304 or T-600 stainless steel) placed adjacent to the thin aluminum separator sheets in addition to on the top and bottom of the stack. Many press loads have at least three steel plates added to the lay-up, which then reduces the number of panels that can be laminated in each press cycle. Some of the lay-up configurations have both aluminum sheets and steel plates separating every panel in the press, with the aluminum separator sheets being discarded after the press cycle. This approach, however, has not completely cured the problem as it causes a decrease in the production rate of the press. Also, pits, dents and other surface imperfections due to the re-introduction of steel plates into the process are still causing scrap and rework of PCB panels. Moreover, many PCB fabricators have to purchase additional new 0.062 stainless steel plates and again install expensive plate cleaning and handling systems. Although the thin aluminum separator sheets are discarded after every press cycle, the steel plates must be cleaned before each use, adding additional operational steps and cost to the PCB fabrication process. To maintain production demands, fabricators must purchase additional vacuum presses, at a cost of approximately $250,000 to $1,000,000 per unit, to compensate for the loss of productivity due to the re-introduction of steel plates into the PCB fabrication process.
Today, fabricators are typically producing between 3 and 8 PCB panels on high technology xe2x80x9cdensexe2x80x9d boards and encounter more quality problems and higher costs. State of the art dense PCB""s now require 2 separators including a 0.062 stainless steel plate and a thin sheet of aluminum. This is an expensive step backward to the beginning of the evolution of the PCB fabrication process.
Use of a thin piece of aluminum in a copper/aluminum laminate structure simply does not meet today""s demanding requirements for high technology, dense PCB""s. Such laminates suffer from a number of drawbacks that include the susceptibility to print through and image transfer, misregistration, blistering, warpage and delamination. In addition, these laminates exhibit unacceptable surface roughness.
Off contact printing often results from image transfer. This generally inhibits the adhesion of dry film and the ability to expose a one to one image on panels. As a result, such laminates are typically limited to the fabrication of four to six layer PCB""s. In addition, shims are often required between every PCB panel. The use of shims adds significant cost to PCB manufacture. The shims must go through a labor intensive cleaning process between each use. Shims are very expensive and many PCB manufacturers have had to set aside space in their manufacturing facilities for shim cleaning.
Misregistration results from too much movement in the inner layers. This causes drill breakage and renders the PCB useless. Drill breakage also results from misregistration in high technology PCBs where small holes which are less 13 mils and as small as 4 mils are typical.
Blistering results from the uneven coefficient of thermal expansion exhibited by aluminum. The uneven CTE creates more hot spots that cause blistering. This problem may not become apparent for six months or more after fabrication and, therefore, may cause major system failures.
Surface roughness is also a problem with aluminum. The high surface roughness will cause off contact printing, broken drill bits, and loss of materials. The laminates are also susceptible to warpage, which renders them useless. And, delamination has been observed using the laminates at low pressures.
Accordingly, there is a need for a copper/metal laminate for use in manufacturing printed circuit boards that exhibits reduced incidence of misregistration, warpage, blistering, image transfer and other undesirable characteristics. The present invention addresses that need, as well as others, and overcomes deficiencies found in conventional copper/metal laminates used for manufacturing printed circuit boards.
The deficiencies inherent in conventional PCB laminates and lamination processes are overcome by the laminate structure of the present invention which generally comprises a sheet of metal, preferably other than aluminum or copper, that is attached to a sheet of copper. The present invention is an extension of the laminated sheet structure and method of attaching the sheets described in my prior U.S. Pat. Nos. 6,129,990 and 6,127,051 which are incorporated herein by reference.
By way of example, and not of limitation, the present invention preferably comprises a steel alloy layer having a corrosive resistant coating and a copper foil layer that is attached to the steel alloy layer. Attachment of the cooper and steel is preferably by means of a combination of adhesive and resistance welds. The adhesive is preferably positioned in strips or lines on one set of opposing sides of the board and the resistance welds are preferably positioned on a second set of opposing sides. In this way, a series of spaced apart resistance welds joins the materials along at least a first perimeter edge, and the adhesive joins the materials along at least a second perimeter edge that is adjacent to the first.
The adhesive portion of the attachment can be facilitated in any conventional manner using, for example, adhesive materials described in used in U.S. Pat. No. 5,153,050 incorporated herein by reference, U.S. Pat. No. 5,120,590 incorporated herein by reference, or U.S. Pat. No. 5,512,381 incorporated herein by reference. Further, it does not matter whether the adhesive is flexible or rigid. It will be appreciated, however, that the adhesive should be of a type that either releases the metal layers at press temperatures or otherwise does not interfere with the subsequent separation of the metal layers.
The resistance welding portion of the attachment can be facilitated by making contact bonds between the surfaces of the metal materials, generally in the form of circular-shaped welds of approximately 4-mils to 20-mils in diameter. Passing an electrical current through the metal layers to be joined generates each weld.
In the present invention, the copper foil layer is attached to either one or both sides of the steel layer. When heated and compressed onto the PCB panel, the copper adheres to and becomes a functional element of the final PCB. The steel layer is subsequently removed and discarded. Use of the steel layer of the present invention eliminates and/or reduces the need for conventional 0.062 stainless steel plates in the press lay-up. As a result, the surface quality of the PCB is improved, image transfer is reduced and a flatter, less wavy PCB is produced without the cost of using separate 0.062 stainless steel separator plates. Furthermore, the similarity in the coefficients of thermal expansion (CTE) between steel in accordance with the present invention and copper allows the PCB""s produced to be flatter, thus providing for a better registration of the foil layer to the substrate layer of the sheet laminate.
Additional information regarding sheet laminate fabrication technology and processes for manufacturing PCB""s can be found in U.S. Pat. Nos. 4,875,283, 5,120,590, 5,512,381 and 5,153,050, each of which is incorporated herein by reference.
The copper/steel laminate structure of the present invention also exhibits superior performance over copper/aluminum, copper/copper, and other known copper/metal laminate structures. For example, with the laminate structure of the present invention it is possible to manufacture PCBs having up to eighteen layers without the need to use shims.
Because the steel sheet used in the present invention does not exhibit the image transfer problems associated with aluminum/copper and copper/copper laminates, the invention also allows more PCBs per book as compared to conventional laminates thereby facilitating increased production. In addition, the steel layer allows PCBs to be run at higher pressures, thereby providing a flatter and more usable PCB area. Furthermore, the need for conventional 0.062 stainless steel separator plates is eliminated, with a resulting reducing in material and facility cost.
The steel layer used in the present invention exhibits a more stable heat rise than aluminum, thereby reducing the possibility of misregistration and drill breakage. The similarity in the coefficients of thermal expansion (CTE) between steel and copper allows the PCB""s produced to be flatter, thus providing a better registration of the foil layer to the substrate layer of the sheet laminate.
Blistering and warpage are also reduced because the CTE of the metal substrate of the present invention is similar to that of copper. And, the smooth surface of the steel of the present invention promotes the fabrication of high density PCBs. Furthermore, the delamination problems associated with aluminum/copper and copper/copper laminate structures are eliminated.
An object of the invention is to provide a sheet laminate, for use in a press lay-up between PCB layers that eliminates the need for aluminum separator sheets and their associated problems.
Another object of the invention is to reduce the amount of waste in PCB manufacturing.
Another object of the invention is to eliminate the surface quality and image transfer problems associated with aluminum separator sheets.
Another object of the invention is to eliminate the need for constant scrubbing and cleaning of conventional 0.062 stainless steel separator plates.
Another object of the invention is to provide a sheet laminate, for use in a press lay-up between PCB layers that eliminates the need for conventional 0.062 stainless steel plates and their associated problems.
Another object of the invention is to provide a sheet laminate, for use in a press lay-up between PCB layers that reduces and/or eliminates image transfer from the substrate layer onto the foil layer.
Another object of the invention is to provide a sheet laminate, for use in a press lay-up between PCB layers, that results in flatter, less wavy PCB panels.
Another object of the invention is to provide a sheet laminate for use in a press lay-up between PCB layers that has a CTE less than or equal to the CTE of copper.
Still another object of the invention is to provide a sheet laminate, for use in a press lay-up between PCB layers, that maximizes the number of PCB panels that can be produced for a given press opening.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.